Bruce WhitefieldMentor, Team 2471

Manipulators for FIRST FRC Robotics

FIRST Fare 2012

Game pieces come in many sizes and shapes

Manipulate What ?

Game objectives change each yearManipulate How ?

Kicking

Lifting Dumping

Hanging

ThrowingGathering

Know the Objectives

Game pieces

Game & robot rules

Your game strategy

Things that Manipulate

Manipulators that work for the game

Know the possibilities

Look on line, at competitions

Talk to mentors, teams

This seminar

Know your capabilities

Tools, Skills, Materials,

Manpower , Budget , Time Manipulators you can build

Your Design

Stuff That Don’t Work

Don’t be this team

Heavy lifting / Long reacho Articulating Armso Parallel armso Telescoping Lifts

Gripperso Rollerso Clampso Claws

Collect and Delivero Accumulators & Conveyerso Turretso Shooters & Kickerso Buckets & Tables

Power & Controlo Wincheso Brakeso Latcheso Pneumaticso Springs and Bungeeo Gears & Sprockets

Many Types of ManipulatorsReoccurring

themes in FIRST games

FIRST definition for a manipulator:

Device that moves the game piece from where

it is to where it needs to be

ShoulderElbowWrist

Arms

Torque = Force x Distanceo Same force, different angle = different torqueo Measure from the pivot pointo Motor & gearing must overcome torqueo Maximum torque at 90 degrees

W=10 lbs

W= 10 lbs

1/2 D

D

Torque = W x D

Torque: Angle and Distance

Torque = W x D/2

Power determines how fast you can move thingsPower = Torque / Time or Torque x Rotational VelocitySame torque with 2x Power = 2x SpeedOr twice the arm length at same speed ….

10 lbsPower trade offs

10 lbs30 ft-lbs

20 lbs60 ft-lbs

425 Watts

100 RPM1.6 rps

50 RPM.83 rps

42.5 Watts

10 RPM.16 rps

5 RPM.08 rps

3 ft

Power limits arm length.

Probably don’t want a 10 ft arm whipping around at 1000 RPM anyway…

Power: Torque and Speed

Lightweight Materials: tubes, thin wall Watch elbow and wrist weight at ends of armSensors for feedback & control

limit switches, potentiometers, encodersLinkages help control long armsKISS your arm

Less parts… to build or break Easier to operate More robust

Counterbalance Spring, weight, pneumatic, bungee…

Calculate the forces Watch out for Center of gravity Sideways forces when extended

Model the reach & orientation

Arm Design Tips

Extension Lifts Motion achieved by stacked members sliding on each other

Scissor Lift Motion achieved by “unfolding” crossed members

Telescoping LiftsTelescoping Lifts

Continuous Cascade

Extension Lift RiggingExtension Lift Rigging

The final stage moves up first and down last. Previous stage speed plus own speed

o Power vs. speed equations apply. Don’t underestimate the power.

o Often needs brakes or ratchet mechanism

Up-pulling and down-pulling cables have different speeds

Different cable speeds can be handled with different drum diameters or multiple Pulleys

Intermediate sections don’t jam – active return

High tension on the lower stage cables

Slider(Stage3)

Stage2

Stage1

Base

Cascade RiggingCascade Rigging

Cable goes same speed for up and down pulling cables

Intermediate sections sometimes jam

Lower cable tensionMore complex cable routing

Stage2

Stage1

Base

Continuous RiggingContinuous RiggingSlider

(Stage3)

Pull-down cable routed back on reverse route as pull-up cable

Most complex cable routingAll stages have active returnCleaner and protected cables

Slider(Stage3)

Stage2

Stage1

Base

Continuous Rigging InternalContinuous Rigging Internal

Use cables to drive up AND down, or add a cable recoil device

Segments must move freelyCable lengths must be adjustableMinimize slop and free-playMaximize segment overlap

20% minimum More for bottom, less for top

Stiffness and strength neededHeavy system, overlapping partsMinimize weight, especially at top

Extension Lift Design tipsExtension Lift Design tips

FeatureFeature ArmArm LiftLiftReach over objectReach over object YesYes NoNo

Fall over, get upFall over, get up Yes, if strong enoughYes, if strong enough NoNo

ComplexityComplexity ModerateModerate HighHigh

Weight capacityWeight capacity ModerateModerate HighHigh

Go under barriersGo under barriers Yes, fold downYes, fold down Maybe, limits lift heightMaybe, limits lift height

Center of gravity ()Center of gravity () CantileveredCantilevered Central massCentral mass

Operating spaceOperating space Large swing spaceLarge swing space CompactCompact

Adding reachAdding reach Difficult. More Difficult. More articulations articulations

Easier. Easier.

More lift sectionsMore lift sections

CombinationsCombinations Arm with extenderArm with extender Lift with arm on topLift with arm on top

Arms vs. LiftsArms vs. Lifts

Combination Example: Combination Example: Continuous direct drive chain

runs stage 1 up and down No winch drum needed

Cascade cable lifts slider stage Gravity return Got away with this only

because slider GOG very well balanced on first stage

Telescoping arm with wrist on slider stage to reach over objects

2471 in 2011

FIRST definition of a gripper:

Device that grabs a game object

…and releases it when needed.

Get a GripGet a Grip

MethodsPneumatic claws /clamps

1 axis2 axis

Motorized claw or clampRollersHoop gripsSuction

Design ConcernsGetting object into gripHanging onSpeed of grip and releasePosition controlWeight and power source

If at end of arm

PneumaticOne fixed armreduce weight of clawCan make one or two

moving sides

768 in 2008768 in 2008

Claw or clampClaw or clamp

Pneumatic Cylinder extends & retracts linkage to open and close gripper

Combined arm and gripperEasy to manufactureEasy to controlQuick grabLimited grip forceUse 3 fingers for 2-axis grip

968 in 2004968 in 2004

Pneumatic: 2 and 3 point clampsPneumatic: 2 and 3 point clamps

60 in 200460 in 2004

Generally slower May be too slow for

frequent grips Okay if few grabs per

game of heavy objects

More complex (gearing & motors)

HeavierTunable forceNo pneumatics

49 in 200149 in 2001

Motorized clampMotorized clamp

Needs vacuum generatorUses various cups to grabSlowNot secureNot easy to controlSimpleSubject to damage of

suction cup or game pieces

Not recommended for heavy game pieces

Used successfully to hold soccer balls for kickers (Breakaway 2010)

Suction GripsSuction Grips

Allows for misalignment when grabbing

Won’t let goExtends object as releasingSimple mechanismHave a “full in” sensorMany possible variations

Mixed roller & conveyer Reverse top and bottom roller

direction to rotate object

45 in 200845 in 2008

148 in 2007148 in 2007

Roller gripsRoller grips

Counter Rotating Methods

Many ways to achieve counter rotating shafts. Here are few configurations that can run off a single motor or gearbox.

Could also drive each shaft with own motor

Crossed round belts

Counter rotating gearbox

Stacked pulleys on single drive shaft

Hang On! High friction needed

Rubber, neoprene, silicone, sandpaper … but don’t damage game object Force: Highest at grip point

Force = 2 to 4 x object weight Use linkages and toggles for mechanical advantage

Extra axis of grip = More control

The need for speed Wide capture window Quickness covers mistakes

Quick to grab , Drop & re-grab Fast : Pneumatic gripper. Not so fast: Motor gripper

Make it easy to control Limit switches , Auto-functions Intuitive control functions

Don’t make driver push to make the robot pull

Gripper design Gripper design

Tube or post (recommended)Lazy Susan (not for high loads)Know when it is needed

o One Goal = goodo Nine Goals = not so good o Fixed targets = goodo Moving targets = not so good

Bearing structure must be solidRotation can be slowInclude sensor feedback

o Know which way it is pointing

Rotating TurretsRotating Turrets

Accumulator: Rotational device that collects objectsHorizontal rollers: gathers balls from floor or platformsVertical rollers: pushes balls up or downWheels: best for big objectsCan use to dispense objects out of robot

Pointing the robot is aiming method in this case

Point to point within your robotWhen game involves handling

many pieces at once

Why do balls jam on belts?- Stick and rub against each other as they

try to rotate along the conveyor

Solution #1- Use individual rollers- Adds weight and complexity

Solution #2- Use pairs of belts- Increases size and complexity

Solution #3- Use a slippery material for the non-moving

surface (Teflon sheet works great)

Conveyers: For moving multiple objects

Conveyer roller Examples Conveyer roller Examples BeltsBeltsBeltsBelts TowerTowerTowerTowerRollersRollersRollersRollers

Solution 1Solution 1Solution 1Solution 1 Solution 3Solution 3Solution 3Solution 3Solution 2Solution 2Solution 2Solution 2

More control is betterAvoid gravity feeds – these are slow and will jamDirect the flow: reduce “random” movements.

Not all game objects are created equalTend to change shape, inflation, etcBuilding adaptive/ flexible systemsTest with different sizes, inflation, etc.

Speed vs. VolumeOptimize for the game and strategyThe more capacity, the better

Intake Rollers and Accumulators

173 2471

Secure shooting structure = more accuracy Feed balls individually, controlling flowRotating tube or wheel

One wheel or two counter rotating High speed & power: 2000-4000 rpm Brace for vibration Protect for safety

Turret allows for aimingSensors detect ball presence

& shot direction

1771 in 20091771 in 2009

Note Circular Note Circular Conveyer. One Conveyer. One roller on inside roller on inside cylindrical rolling cylindrical rolling surface outsidesurface outside

Use for dumping many objectsIntegrate with your accumulator

and conveyerHeavy ones may move too slowUsually pneumatic powered but

can run with gear, spring or winch

488 in 2009488 in 2009

Many uses Hanging Robots: 2000, 2004, 2010 Lifting Robots: 2007 Loading kickers 2010

Great for high torque applicationsFits into limited spaceEasy to route or rerouteGood Pull. Bad Push

20042004

20102010

Secure the cable routing Smooth winding & unwinding Leave room on drum for wound up cable Guide the cableMust have tension on cable to unwind

Can use cable in both directions Spring or bungee returnGravity return (not recommended)

Calculate the torque and speedUse braking devices

Capture Capture the the cablecable

Maintain Maintain TensionTension

Guide Guide the the cablecable

Brake or ratchetBrake or ratchet

Retu

rn c

able

or

spri

ng

Retu

rn c

able

or

spri

ng

Sudden release of powerUse stored energy:

Springs Bungee, PneumaticDesign & test a good latch

mechanismSecure lock for safetyFast release

1 per game deployments2011 minibot release

Hooking and latching devices used to grab goals, bars, and other non-scoring objects

Hold stored power in place Spring or bungee power

Several ways: Hooks Locking wheels Rollers Pins Springs

Self latching wheel lockSelf latching wheel lock

Basic Automatic LatchBasic Automatic LatchBasic Automatic LatchBasic Automatic Latch

Strong pivot in Strong pivot in line with large line with large force being heldforce being held

Strong pivot in Strong pivot in line with large line with large force being heldforce being held

Spring Spring return and return and stopstop

Spring Spring return and return and stopstop

Small release force Small release force at end of leverat end of lever

Small release force Small release force at end of leverat end of lever

Bevel for self latchingBevel for self latchingBevel for self latchingBevel for self latching

Removable safety pinRemovable safety pinRemovable safety pinRemovable safety pin

Start design early. Latches tend to be afterthoughts but are often a critical part of the operation

Don’t depend on driver to latch, use a smart mechanism Spring loaded (preferred) Sensor met and automatic command given Use operated mechanism to let go, not to latch

Have a secure latchDon’t want release when robots crash

Be able to let go quickly Pneumatic lever Motorized winch, pulling a string Cam on a gear motor Servo (light release force only)

Don’t forget a safety pin or latch for when you are working on the robot

Look around see what works and does not workKnow your design objectives and game strategyStay within your capabilitiesDesign before you build

Calculate the forces and speeds Understand the dimensions using CAD or a model

Keep it simpleMake it well

Poor craftsmanship can ruin the best design

Analyze for failure points. Use to refine design and decide on spare parts

Have fun

Many thanks to teams and companies who made materials for this presentation freely available on web sites to help FIRST students. Andy Baker : Team 45Jason Marr : Team 2471Wildcats: Team 1510The Flaming Chickens: Team 1540Society of robots

Andy Baker’s original presentation and inspiration for this seminar is available on line

There are many examples and resources available. Be sure to use them for your robot designs.

http://www.societyofrobots.com/mechanics_gears.shtmlhttp://www.societyofrobots.com/mechanics_gears.shtml

Appendix

Assuming 100% power transfer efficiency:All motors can lift the same amount they just do it at

different rates.No power transfer mechanisms are 100% efficient

Inefficiencies due to friction, binding, etc. Spur gears ~ 90% Chain sprockets ~ 80% Worm gears ~ 70% Planetary gears ~80%

Calculate the known inefficiencies and then design in a safety factor (2x to 4x)

Stall current can trip the breakers

Motor Power:

It adds up! 2 spur gears + sprocket =.9 x.9 x.8 = .65 Losing 35% of power to the drive train

• Pin loading can be very high• Watch for buckling in lower arm• Has limited range rotation• Keeps gripper in fixed orientation

Parallel Arms

Advantages Minimum retracted height - can go

under field barriersDisadvantages

Tends to be heavy when made stable enough

Doesn’t deal well with side loads Must be built very precisely Stability decreases as height

increases Stress loads very high at beginning

of travel

Not recommend without prior experience

Scissor LiftsScissor Lifts

Pneumatic latch, solidly grabs pipe

Force and friction onlyNo “smart mechanism”

Spring-loaded latchMotorized releaseSmart Mechanism

469 in 2003469 in 2003

Parallel arm Fixed Arm

Jointed Arm

Ratchet - Complete lock in one direction in discrete increments Clutch Bearing - Completely lock in one direction any spot Brake pads - Squeezes on a rotating device to stop motion - can lock in

both directions. Simple device Disc brakes - Like those on your mountain bike Gear brakes - Apply to lowest torque gear in gearbox Belt Brake- Strap around a drum or pulley

Dynamic Breaking by motors lets go when power is lost. Use for control, but not for safety or end game Gearbox that cannot be back-driven is usually an inefficient one.